Method and system for the separation of a mixture containing carbon dioxide, hydrocarbon and hydrogen

ABSTRACT

The invention provides a method and system for the separation of a mixture containing H2, hydrocarbon, and C02. The generated mixture ( 3 ) is introduced into a distillation column ( 2 ) having a side stream to generate a top stream comprising H2 ( 21 ), a middle (volatility) stream comprising hydrocarbon ( 30 ); and a bottom stream comprising C02 ( 32 ). In a preferred embodiment, the mixture further comprises N2 which is obtained in the top stream ( 21 ). In a most preferred embodiment, the H2 and the N2 are present in a molar ratio of 3H2:1N2. The generated H2 and N2 may be used for the synthesis of ammonia. Thus, the invention also proves a method and system for the generation of ammonia.

FIELD OF THE INVENTION

This invention relates to methods and systems for the separation of mixtures containing carbon dioxide, hydrocarbon, and hydrogen. The invention may be used, for example, for the separation of a syngas to produce hydrogen and nitrogen which may be used in the synthesis of ammonia.

BACKGROUND OF THE INVENTION

The term “synthesis gas”, also known as “syngas” refers to a gas mixture containing carbon dioxide and/or monoxide and molecular hydrogen generated by the gasification of a carbon-containing fuel to a gaseous product with a heating value. Syngas is produced, for example, by steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal and in some types of waste-to-energy gasification facilities. Syngas is used, for example, as intermediates in creating synthetic natural gas, and for producing ammonia or methanol. Syngas is also used as an intermediate in producing synthetic petroleum for use as a fuel or lubricant.

In the synthesis of ammonia from stoichiometric air (as a source of nitrogen) and hydrocarbons (as a source of hydrogen), the hydrocarbon, such as methane, is made to react with steam at elevated temperatures to generate H₂, CO, CO₂H₂O. This produces a raw syngas containing, in addition to these compounds, residual unreacted hydrocarbon, as well as N₂, and other air constituents. The N₂ and H₂ must then be separated from the other components of the syngas for the generation of ammonia.

There are two main problems in the production of ammonia. One problem relates to the fact that excessive amounts of hydrocarbon, typically methane, remain unreacted in the conversion of the hydrocarbon to H₂ and CO, so that the ammonia yield is far from optimal. This so-called hydrocarbon slip can be reduced by using high reforming temperatures. Secondly, CO₂ must be removed from syngas to prevent poisoning of the catalyst used in the ammonia conversion, and this CO₂ removal requires high capital costs and is also costly in terms of energy consumption.

Significant work has been applied to the development of methods for the removal of carbon dioxide from a syngas. The processes can be separated into four general classes; absorption by physical solvents, absorption by chemical solvents, adsorption by solids, and distillation.

The high relative volatility of methane with respect to carbon dioxide makes cryogenic distillation theoretically very attractive. However, the methane/carbon dioxide distillative separation has a significant disadvantage in that solid carbon dioxide exists in equilibrium with vapor-liquid mixtures of carbon dioxide and methane at particular conditions of temperature, pressure, and composition. Obviously, the formation of solids in a distillation tower has the potential for plugging the tower and its associated equipment. Increasing the operating pressure of the tower will result in warmer operating temperatures and a consequent increase in the solubility of carbon dioxide, thus narrowing the range of conditions at which solid carbon dioxide forms. However, additional increases in pressure will cause the carbon dioxide-methane mixture to reach and surpass its critical conditions. Upon reaching criticality, the vapor and liquid phases of the mixture are indistinguishable from each other and therefore cannot be separated. A single-tower equilibrium separation operating in the vapor-liquid equilibrium region bounded between carbon dioxide freezeout conditions and the carbon dioxide-methane critical pressure line may produce a product methane stream containing 10% or more carbon dioxide.

Various methods have been devised to avoid the conditions at which carbon dioxide freezes and yet obtain an acceptable separation. Two processes which utilize additives to aid in the separation are disclosed in U.S. Pat. No. 4,149,864 to Eakman et al, and U.S. Pat. No. 4,318,723 to Holmes et al.

Eakman et al discloses a process for separating carbon diokide from methane in a single distillation column. If insufficient hydrogen is present in the column feedstream, hydrogen is added to provide a concentration from about 6 to 34 mole percent, preferably from about 20 to about 30 mole percent. The separation is said to take place without the formation of solid carbon dioxide. The tower pressure is preferably held between 1025 and 1070 psia.

Holmes et al adds alkanes having a molecular weight higher than methane, preferably butane, to the tower feed to increase the solubility of carbon dioxide and decrease its freezing temperature line. The additive n-butane is added at an amount from about 5 moles to 30 moles per 100 moles of feed.

U.S. Pat. No. 4,511,382 to Valencia et al discloses separating acid gases, particularly carbon dioxide, from methane by cryogenic distillation in which an effective amount of a light gas, preferably helium, is added to a stream containing methane and carbon dioxide and cryogenically distilling the mixed stream to produce a liquid carbon dioxide stream and an enriched methane stream. The distillation tower or at least a portion thereof may then be operated at a pressure higher than the critical pressure of methane.

A process for the separation of carbon dioxide from a predominantly methane stream is described in U.S. Pat. No. 2,888,807 to Bocquet. The separation requires the use of two distillation columns arranged in series. When the carbon dioxide is present at a concentration below 8 mole percent, the feed is introduced into the first column of the series, and where the carbon dioxide is present at concentration above 8 mole percent, the feed is introduced directly into the second column of the series. The first column is operated at or below the critical temperature of methane such that feed to each column provides a carbon dioxide concentration below which, on cooling at the operating pressure of the column, would produce a solid carbon dioxide phase. Effluents from the top of the second column contain substantially the same concentration of carbon dioxide as the feeds to the first columns. The operating pressure applied to the second column is maintained above a critical pressure defined as that at which the carbon dioxide phase will exist, and above which pressure a solid carbon dioxide phase will not coexist with a vapor.

U.S. Pat. No. 7,090,816 to Malhotra et al discloses a method for the purification of syngas, such as occurs in the manufacture of ammonia, using cryogenic distillation. Refrigeration for the distillation is obtained from waste fluid expansion using a liquid expander to recover mechanical work from the waste fluid. This method reduces pressure loss in the syngas stream and reduces compression and power relative to similar ammonia generating processes.

SUMMARY OF THE INVENTION

In one of its aspects, the invention provides a method for the separation of a mixture containing H₂, hydrocarbon, and CO₂. In accordance with this aspect of the invention, the mixture is introduced into a distillation column having a side stream. Distillation of the mixture using a column having a side stream generates three streams, as follows:

-   -   (i) a top stream comprising H₂;     -   (ii) a middle stream comprising hydrocarbon; and     -   (iii) a bottom stream comprising CO₂.

In a preferred embodiment of the method and system of the invention, the mixture further comprises N₂. In a most preferred embodiment, the H₂ and N₂ are present in a molar ratio of 3:1. This may be achieved by mixing the hydrocarbon with a stoichiometric amount of air, as is known in the art. Embodiments in which the H₂ and N₂ are present in this molar ratio are useful for generating H₂ and N₂ for use in the manufacture of ammonia. Thus, in another of its aspects, the invention provides a method and system for the production of ammonia. In accordance with this aspect of the invention, a mixture containing H₂, N₂, hydrocarbon, and CO₂ is introduced into a distillation column to produce the three streams described above. The top stream comprising H₂ and N₂ is then used to generate ammonia by any method known in the art.

The process has several degrees of freedom allowing flexibility in determining the operating parameters of the system, such as methane slip and side stream composition, and can improve the efficiency of the raw syngas generating process.

Thus, in one of its aspects, the present invention provides a method for the separation of a mixture containing H₂, hydrocarbon, and CO₂, the method comprising introducing the mixture into a distillation column having a side stream to generate:

-   -   (i) a top stream comprising H₂;     -   (ii) a middle (volatility) stream comprising hydrocarbon; and     -   (iii) a bottom stream comprising CO₂.

The hydrocarbon is preferably, although not necessarily, methane. In a preferred embodiment of the method of the invention, the mixture further comprises N₂ which is obtained by the method in the top stream. In a most preferred embodiment, the H₂ and the N₂ are present in the mixture in a molar ratio of 3H₂:1N₂. Thus, in another of its aspects, the invention provides a method for generating ammonia.

In another of its aspects, the invention provides a system for generating and separating a mixture containing H₂, hydrocarbon, and CO₂, the system comprising:

-   -   (a) means for generating a mixture containing H₂, hydrocarbon,         and CO₂; and     -   (b) a distillation column having a side stream configured to         receive the mixture and to generate from the mixture:         -   (i) a top stream comprising H₂;         -   (ii) a middle stream comprising hydrocarbon; and         -   (iii) a bottom stream comprising CO₂.

The means for generating a mixture containing H₂, hydrocarbon and CO₂, may be any such system known in the art.

The hydrocarbon of the system is preferably, although not necessarily, methane. In a preferred embodiment of the system of the invention, the mixture further comprises N₂ which is obtained by the method in the top stream. In a most preferred embodiment, the H₂ and the N₂ are present in the mixture in a molar ratio of 3H₂:1N₂. Thus, in yet another of its aspects, the invention provides a system for generating ammonia.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawing, in which:

FIG. 1 shows a system for separating a mixture containing H₂, hydrocarbon, and CO₂, in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a system 1 for separating a feed mixture 3 containing H₂, N₂, hydrocarbon and CO₂, in accordance with one embodiment of the invention. The system 1 comprises a distillation column 2, a reboiler 4 and a condenser 18. The distillation column 2 comprises one or more trays 8, as is known in the art for distillation columns. The feed mixture 3 may be a gas, or liquid/gas mixture. The feed mixture 3 is introduced into the distillation column 2 at an inlet 10 and is deposited into a feed tray 12 of the column 2. After the mixture reaches equilibrium in the feed tray 12, the H₂ and N₂, flow up the column while the CO₂ is liquefied and flows to the bottom by gravity. The working pressure inside the column should be above about 5 bar in order to assure that the CO₂ can exist in the liquid phase in the column. Part of the hydrocarbon entering the column is liquefied, mixed with the CO₂, and then flows down towards the bottom of the column. Gaseous hydrocarbon first rises in the column and liquefies by the liquid nitrogen, and then flows down and re-evaporates. In every tray there is a liquid/vapor equilibrium whose composition is determined by the tray's temperature.

The distillation column 2 generates a condenser feed stream 14 containing primarily the H₂ and N₂ in gaseous form. The output stream 14 is introduced into a condenser 18 that generates a liquid reflux 20 that returns to the column 2, preferably to the top tray 22 of the column. The reflux of the top stream to the column is preferably performed using a reflux ratio between 0.001 and 10, and more preferably between 0.5 and 2. It is possible to alter the temperature gradient in the column by varying the reflux rate.

The reflux 20 serves as a cooling source inside the column for the trays 8 above the feed tray 12. In the lower part of the column, where the temperature is higher, only small amounts of liquid nitrogen are present, and most of the cooling for the trays 8 below the feed tray 12 is provided by liquid hydrocarbon and liquid CO₂. An overhead product stream 21 containing primarily gaseous H₂ and N₂ is drawn off from the condenser 18. As explained above, the overhead product stream 21 can be used in the synthesis of ammonia. The distillation column 2 also generates a reboiler feed stream 24 containing primarily hydrocarbon and CO₂. The reboiler feed stream 24 is introduced into the reboiler 4. In the reboiler 4, hydrocarbon boils, and may be withdrawn as a vapor side stream 30, while liquid CO₂ is withdrawn as a bottom stream 32. The liquid CO₂ in the bottom stream is easier to dispose of than gaseous CO₂. The reboiler generates a boilup 26 that is returned to the column 2 preferably to the bottom tray 28 of the column 2. The vapor side stream 30 can be recycled and reformed and used to generate new feed stream 3. Recycling of the hydrocarbon increases the utilization of the hydrocarbon, thus increasing the ammonia yield.

The column preferably has a pressure between 5 bar to a critical pressure of the mixture, and more preferably between 7 bar to 55 bar.

In an alternative embodiment, (not shown), a side stream is withdrawn from a tray 8 in the column 2, instead of withdrawing the side stream 30 from the reboiler 4. In another embodiment, the feed stream 3 is introduced directly into the reboiler which is set to conditions under which CO₂ is a liquid at its bubble point and is withdrawn.

EXAMPLES

The method and system of the invention was implemented on the process simulator UniSim Design Version R370Build 13058 of Honeywell.

Example 1

In this example, the operating parameter values shown in Table 1 were used in the simulation. The thermodynamic package used was the Peng Robinson Sour Vapor package.

TABLE 1 Syn loop pressure 15 bar primary reforming temperature 700° C. Number of trays 12 plus condenser and reboiler inlet temperature −55° C. (feed stream in gaseous state) Location of feed inlet Tray 7 from the bottom Reflux ratio 1.9 Side stream draw stage Reboiler Side stream flow rate 300 kgmole/hour

Table 2 shows in Column (a) the feed stream 3 to the column 2 that was generated by the simulation using the parameters of Table 1. The flow rate (col (b)), molar fraction (col (c)), and the partial pressure (col (d)) of the feed stream are also shown in Table 2.

TABLE 2 Flow rate partial pressure Feed Stream [kgmol/hr] molar fraction - y [atm] (a) (b) (c) (d) CO₂ 654 0.164 2.455 H₂ 2206 0.552 8.280 CH₄ 368 0.092 1.381 N₂ 751 0.188 2.819 Ar 17.59 0.004 0.066 Total 3996.59 1.000 15

Table 3 shows the composition of the processed syngas, or overhead stream 14, as well as its flow rate and the pressure of the stream 14, as determined by the simulation.

TABLE 3 Syngas (overhead Flow rate partial pressure stream): [kgmol/hr] molar fraction - y [atm] Species (b) (c) (d) CO₂ 0 0 0 H₂ 2207 0.748 11.22 CH₄ 0.0033 1 × 10⁻⁶ 1.5 × 10⁻⁵ N₂ 735.6 0.249 3.735 Ar 6.131 0.004 0.06 Total 2948.75 1 15

Table 4 shows the composition of the side stream 30 that was generated.

TABLE 4 Flow rate partial pressure [kgmol/hr] molar fraction - y [atm] Side Stream: (b) (c) (d) CO₂ 5.4 0.018 0.27 H₂ 0 0 0 CH₄ 270.59 0.905 13.575 N₂ 14.03 0.046 0.69 Ar 8.77 0.029 0.435 Total 298.8 1.000 15

Table 5 shows the composition of the bottom stream 32 that was generated.

TABLE 5 Flow rate partial pressure Bottom stream [kgmol/hr] molar fraction - x [atm] (a) (b) (c) (d) CO₂ 648.6 0.865 12.975 H₂ 0 0 0 CH₄ 97.41 0.13 1.95 N₂ 1.372 0.00183 0.0275 Ar 2.682 0.035 0.525 Total 750.064 1.000 15

Table 6 shows the ammonia product yield, the purity of the ammonia yield, the condenser duty, and the reboiler duty.

TABLE 6 ammonia product yield 593.6 ton/day purity of the ammonia yield 0.997 Ammonia temperature −28° C. Ammonia pressure 15 bar

Table 7 shows the energy consumption. Energy costs were calculated assuming a use of a nitrogen refrigerant utility at a cost of 1 million dollar/million kcal/hr/yr. The energy demand and cost were determined by the reflux ratio. When running the column at 15 bar the reflux composition was primarily liquid nitrogen.

TABLE 7 Condenser duty 7.08 × 10⁶ Energy cost of $7.08 kcal/hr Condenser million/year Reboiler duty 1.06610×⁶ Energy cost of $1.0661 kcal/hr Reboiler million/year

Table 8 shows the steady state tray composition profile of the column (molar flows, kgmole/hr).

TABLE 8 CO₂ CH₄ Stage (Vap) H₂ (Vap) (Vap) N₂ (Vap) Ar (Vap) Temperature Condenser 1.01 × 10⁻²⁶ 2208 1.55 × 10⁻⁴ 735.8 5.526 −185.5 12  6.21 × 10⁻²² 2484 1.17 × 10⁻² 5984 84.91 −169.9 11  3.58 × 10⁻¹⁹ 2368  6.8 × 10⁻² 7123 154.7 −168.6 10  1.45 × 10⁻¹⁶ 2347 0.3185 7146 232 −168.4 9 5.76 × 10⁻¹⁴ 2343 1.461 7016 337.8 −168.2 8  2.3 × 10⁻¹¹ 2339 6.671 6809 484.5 −167.8 7  9.3 × 10⁻⁹ 2333 30.32 6486 681.7 −167.2 6  3.7 × 10⁻⁶ 2321 135.6 5896 918.2 −165.3 5  1.3 × 10⁻³ 2290 568.8 4577 1066 −158.5 4 0.2306 2241 1796 2274 747.8 −143.4 3 8.181 2229 2965 1125 281.8 −138.1 2 0.3702 1.156 40.59 13.89 4.67 −123.7 1 2.649 0.1531 206.2 26.98 17.31 −117.5 Reboiler 6.481 3.18 × 10⁻³ 287.4 8.145 11.93 −108.3 CH₄ Stage CO₂ (Liq) H₂ (Liq) (Liq) N₂ (Liq) Ar (Liq) Temperature Condenser  6.2 × 10⁻²² 276.4 1.15 × 10⁻² 5248 79.38 −185.5 12   3.6 × 10⁻¹⁹ 160 6.78 × 10⁻² 6387 149.2 −169.9 11  1.45 × 10⁻¹⁶ 139.2 0.318 6411 226.4 −168.6 10  5.75 × 10⁻¹⁴ 134.8 1.46 6280 332.2 −168.4 9  2.3 × 10⁻¹¹ 131.1 6.67 6073 479 −168.2 8 9.27 × 10⁻⁹  125.3 30.32 5750 676.2 −167.8 7 3.72 × 10⁻⁶  113.1 135.6 5160 912.7 −167.2 6 1.33 × 10⁻³  82.19 568.8 3841 1061 −165.3 5 0.2306 33.09 1796 1539 742.3 −158.5 4 8.181 21.44 2965 388.9 276.2 −143.4 3 655.3 1.59 412.3 22.42 19.54 −138.1 2 657.6 0.1562 578 35.5 32.19 −123.7 1 661.4 6.25 × 10⁻³ 659.1 16.67 26.8 −117.5 Reboiler 648.7 1.99 × 10⁻⁵ 97 0.7374 3.48 −108.3

In this example, the energy consumption of the overall process, 7.15 Gcal/mton ammonia) similar to the energy consumption of existing processes. is the lowest of all the simulations that were performed.

Example 2

In this example, the operating parameter values shown in Table 9 were used in the simulation. The thermodynamic package used was the Peng Robinson package.

TABLE 9 Syn loop pressure 45 bar primary reforming temperature 800° C. Number of trays 10 plus condenser and reboiler Location of feed inlet Tray 6 from the bottom inlet temperature −55° C. (feed stream in gaseous state) Reflux ratio 1.3 Side stream draw stage Tray 4 from the bottom Side stream flow rate 290 kgmole/hour

Table 10 shows in Column (a) the feed stream 3 to the column 2 that was generated by the simulation using the parameters of Table 1. The flow rate (col (b)), molar fraction (col (c)), and the partial pressure (col (d)) of the feed stream are also shown in Table 2.

TABLE 10 Flow rate partial pressure Feed Stream [kgmol/hr] molar fraction - y [atm] (a) (b) (c) (d) CO₂ 716 0.176 7.92 H₂ 2266 0.556 25 CH₄ 317 0.078 3.51 N₂ 763 0.187 8.415 Ar 9.5 0.0023 0.103 Total 4071.5 1 45

Table 11 shows the composition of the processed syngas, or overhead stream 14, as well as its flow rate and the pressure of the stream 14, as determined by the simulation.

TABLE 11 Syngas (overhead Flow rate partial pressure stream): [kgmol/hr] molar fraction - y [atm] Species (b) (c) (d) CO₂ 9.683 × 10⁻⁵ 3.11 × 10⁻⁸ 1.39 × 10⁻⁶ H₂ 2266 0.729 32.805 CH₄ 77 0.025 1.125 N₂ 755.2 0.243 10.935 Ar 9.088 0.0029 0.1305 Total 3107 1 45

Table 12 shows the composition of the side stream 30 that was generated.

TABLE 12 Flow rate partial pressure [kgmol/hr] molar fraction - y [atm] Side Stream: (b) (c) (d) CO₂ 53.727 0.185 8.325 H₂ 0 0 0 CH₄ 236.94 0.815 36.675 N₂ 8 0.0275 1.2375 Ar 0 0 0 Total 290.667 1 45

Table 13 shows the composition of the bottom stream 32 that was generated.

TABLE 13 Flow rate partial pressure Bottom stream [kgmol/hr] molar fraction - x [atm] (a) (b) (c) (d) CO₂ 662.7 0.995 44.775 H₂ 0 0 0 CH₄ 3.077 4.62 × 10⁻³ 0.2 N₂ 0.0065 9.76 × 10⁻⁶  4.4 × 10⁻⁴ Ar 0.003  4.5 × 10⁻⁵ 2.02 × 10⁻⁴ Total 665.75 1 45

Table 14 shows the ammonia product yield, the purity of the ammonia yield, the condenser duty, and the reboiler duty.

TABLE 14 ammonia product yield 617.8 ton/day purity of the ammonia yield 0.977 Ammonia temperature −28° C. Ammonia Pressure 15 bar

Table 15 shows the energy consumption. Energy costs were calculated assuming a use of a nitrogen refrigerant utility at a cost of 1 million dollar/million kcal/hr/yr. For the reboiler, refrigerated brine was used as a utility at a cost of $4×10⁴/million kcal/hr/yr. The energy demand and cost were determined by the reflux ratio. When running the column at 45 bar the reflux composition was primarily liquid nitrogen and methane.

TABLE 15 Condenser duty 6.582 × 10⁶ Energy cost of $6.58 kcal/hr Condenser million/year Reboiler duty 2.872 × 10⁶ Energy cost of $0.13 kcal/hr Reboiler r million/year

Table 16 shows the steady state tray composition profile of the column molar flows (kgmole/hr).

TABLE 16 CO₂ CH₄ Stage (Vap) H₂ (Vap) (Vap) N₂ (Vap) Ar (Vap) Temperature Condenser 2.48 × 10⁻⁵ 1938 43.7 646.3 7.661 −166.1 10  5.17 × 10⁻² 2196 1390 2685 57.69 −126.2 9 0.7223 2083 2645 1772 47.57 −114.5 8 4.416 2071 3256 1203 31.33 −110.3 7 20.66 2072 3464 995.6 21.79 −108.2 6 88.52 2062 3311 910.9 16.67 −104 5 2.862 4.929 41.23 8.343 0.2172 −76.69 4 37.77 4.351 388.4 33.9 1.481 −55.54 3 100.2 1.562 962.4 40.67 2.652 −16.26 2 113.7 0.2723 1048 21.95 2.071 0.9492 1 122.2 4.15 × 10⁻² 986.6 10.2 1.385 6.963 Reboiler 166.3  2.7 × 10⁻³ 433.5 2.231 0.4723 9.122 CH₄ Stage CO₂ (Liq) H₂ (Liq) (Liq) N₂ (Liq) Ar (Liq) Temperature Condenser 5.17 × 10⁻² 257.4 1346 2038 50.03 −166.1 10  0.722 145.1 2601 1126 39.91 −126.2 9 4.416 132.5 3212 556.6 23.67 −114.5 8 20.66 134 3420 349.3 14.13 −110.3 7 88.52 123.9 3267 264.6 9 −108.2 6 596.1 8.019 450.2 32.75 1.392 −104 5 631 7.441 797.4 58.31 2.656 −76.69 4 666.6 1.562 1096 41 2.775 −55.54 3 680.1 0.2724 1181 22.3 2.194 −16.26 2 688.6 4.16 × 10⁻² 1120 10.54 1.508 0.9492 1 732.8 2.83 × 10⁻³ 566.6 2.564 0.5954 6.963 Reboiler 566.4 1.23 × 10⁻⁴ 133.1 0.3323 0.1231 9.122

A phase diagram for the five component mixture of this invention is unavailable in the literature. However, from an analysis of the phase diagram of the corresponding binary system (CO₂/CH₄), and the fact that a high concentration of H₂ leads to an increase of the critical pressure and also to a decrease in the freezing pressure of the CO₂, it can be concluded that under the conditions (pressure and temperature) of this example the working conditions of the system of the present invention in which CO₂ freezing is prevented are broader than those of the binary system.

It is worth noting that for this multi-component mixture, the vapor pressure line of pure CH₄ will not limit the separation boundary at low temperatures. Each of the gasses in the column of the multi-component mixture, other than the methane, has a lower critical temperature than methane.

Example 3

In this example, the input stream was first cooled to a temperature of −100° C. which condenses most of the CO₂ in the feed stream. The cooled feed stream was then passed though a flush allowing most of the CO₂ to be removed from the other components of the feed stream, before being introduced into the column. The operating parameter values shown in Table 17 were used in the simulation. The thermodynamic package used was the Peng Robinson package.

TABLE 17 Syn loop pressure 15 bar primary reforming temperature 700° C. Number of trays 12 plus condenser and reboiler Location of feed inlet Tray 7 from the bottom inlet temperature −100° C. (feed stream in gaseous state) Reflux ratio 0.5 Side stream draw stage reboiler Side stream flow rate 410 kgmole/hour

Table 18 shows in Column (a) the feed stream 3 to the column 2, before passing through the flush, that was generated by the simulation using the parameters of Table 1. The flow rate (col (b)), molar fraction (col (c)), and the partial pressure (col (d)) of the feed stream are also shown in Table 18.

TABLE 18 Flow rate partial pressure Feed Stream [kgmol/hr] molar fraction - y [atm] (a) (b) (c) (d) CO₂ 711.2 0.169 2.535 H₂ 2285 0.542 8.13 CH₄ 447.7 0.106 1.59 N₂ 761 0.18 2.7 Ar 9 0.002 0.003 Total 4213 1 15

Table 19 shows in Column (a) the feed stream 3 to the column 2, after having passed through the flush, that was generated by the simulation using the parameters of Table 1. The flow rate (col (b)), molar fraction (col (c)), and the partial pressure (col (d)) of the feed stream are also shown in Table 19.

TABLE 19 Flow rate partial pressure Feed Stream [kgmol/hr] molar fraction - y [atm] (a) (b) (c) (d) CO₂ 121.3 0.033 0.495 H₂ 2285 0.63 9.45 CH₄ 447.7 0.1235 1.8525 N₂ 761 0.21 3.15 Ar 9 2.48 × 10⁻³ 0.0372 Total 3624 1 15

Table 20 shows the composition of the processed syngas, or overhead stream 14, as well as its flow rate and the pressure of the stream 14, as determined by the simulation.

TABLE 20 Syngas (overhead Flow rate partial pressure stream): [kgmol/hr] molar fraction - y [atm] Species (b) (c) (d) CO₂ 0 0 0 H₂ 2286 0.735 11.025 CH₄ 57.9 0.0186 0.28 N₂ 761.9 0.245 3.675 Ar 9 2.89 × 10⁻⁶ 4.3 × 10⁻⁵ Total 3107 1 15

Table 21 shows the composition of the side stream 30 that was generated.

TABLE 21 Flow rate partial pressure [kgmol/hr] molar fraction - y [atm] Side Stream: (b) (c) (d) CO₂ 30 0.0731 1.1 H₂ 0 0 0 CH₄ 380 0.926 13.89 N₂ 0 0 0 Ar 0 0 0 Total 410 1 15

Table 22 shows the composition of the bottom stream 32 that was generated.

TABLE 22 Flow rate partial pressure Bottom stream [kgmol/hr] molar fraction - x [atm] (a) (b) (c) (d) CO₂ 92.21 0.9 13.5 H₂ 0 0 0 CH₄ 9.764 0.095 1.43 N₂ 0 0 0 Ar 0 0 0 Total from bottom 101.947 1 15 stream CO₂ from flush 589.9 1 15 Total (Flush + Column) 691.874 0.986 15

Table 23 shows the ammonia product yield, the purity of the ammonia yield, the condenser duty, and the reboiler duty.

TABLE 23 Ammonia product yield 619 ton/day purity of the ammonia yield 0.988 Ammonia temperature −28° C. Ammonia pressure 15 bar

Table 24 shows the energy consumption. Energy costs were calculated assuming a use of a nitrogen refrigerant utility at a cost of 1 million dollar/million kcal/hr/yr. The energy demand and cost were determined by the reflux ratio. When running the column at 15 bar the reflux composition was primarily liquid nitrogen.

TABLE 24 Condenser duty 3.466 × 10⁶ Energy cost of $3.466 kcal/hr Condenser million/year Reboiler duty 1.466 × 10⁶ Energy cost of $1.466 kcal/hr Reboiler r million/year

Since the concentration of CO₂ in the column is low due the flushing of the CO2, freezing of any CO₂ in the column will not occur under the pressure 15 bar. This simulation also corresponds to a system in which the cooled feed stream is fed directly into the reboiler.

Example 4

In this example, the operating parameter values shown in Table 1 were used in the simulation. The thermodynamic package used was the SRK package.

TABLE 25 Syn loop pressure 15 bar primary reforming temperature 750° C. Number of trays 10 plus condenser and reboiler Location of feed inlet Tray 6 from the bottom inlet temperature −55° C. (feed stream in gaseous state) Reflux ratio 1.2 Side stream draw stage reboiler Side stream flow rate 199 kgmole/hour

Table 26 shows in Column (a) the feed stream 3 to the column 2 that was generated by the simulation using the parameters of Table 1. The flow rate (col (b)), molar fraction (col (c)), and the partial pressure (col (d)) of the feed stream are also shown in Table 26.

TABLE 26 Flow rate partial pressure Feed Stream [kgmol/hr] molar fraction - y [atm] (a) (b) (c) (d) CO₂ 769 0.189 2.835 H₂ 2330 0.573 8.595 CH₄ 177 0.0435 0.65 N₂ 776 0.191 2.865 Ar 9.36 0.0023 0.0345 Total 4061 1 15

Table 27 shows the composition of the processed syngas, or overhead stream 14, as well as its flow rate and the pressure of the stream 14, as determined by the simulation.

TABLE 27 Syngas (overhead Flow rate partial pressure stream): [kgmol/hr] molar fraction - y [atm] Species (b) (c) (d) CO₂ 1.9 × 10⁻⁹    6 × 10⁻¹³ ~0 H₂ 2330 0.74 11.1 CH₄ 29.84 9.48 × 10⁻³ 0.142 N₂ 776 0.246 3.69 Ar 9.3 2.95 × 10⁻³ 0.04425 Total 3146 1 15

Table 28 shows the composition of the side stream 30 that was generated.

TABLE 28 Flow rate partial pressure [kgmol/hr] molar fraction - y [atm] Side Stream: (b) (c) (d) CO₂ 83 0.417 6.25 H₂ 0 0 0 CH₄ 116 0.583 8.75 N₂ 0 0 0 Ar 0 0 0 Total 199 1 15

Table 29 shows the composition of the bottom stream 32 that was generated.

TABLE 29 Flow rate partial pressure Bottom stream [kgmol/hr] molar fraction - x [atm] (a) (b) (c) (d) CO₂ 685.4 0.956 14.35 H₂ 0 0 0 CH₄ 31.33 0.044 0.65 N₂ 0 0 0 Ar 0 0 0 Total 716.7 1 15

Table 30 shows the ammonia product yield, the purity of the ammonia yield, the condenser duty, and the reboiler duty.

TABLE 30 Ammonia product yield 628.4 ton/day purity of the ammonia yield 0.992 Ammonia temperature −28° C. Ammonia pressure 15 bar

Table 31 shows the energy consumption. Energy costs were calculated assuming a use of a nitrogen refrigerant utility at a cost of 1 million dollar/million kcal/hr/yr. The energy demand and cost were determined by the reflux ratio. When running the column at 15 bar the reflux composition was primarily liquid nitrogen.

TABLE 31 Condenser duty 7.718 × 10⁶ Energy cost of $7.718 kcal/hr Condenser million/year Reboiler duty 1.883 × 10⁶ Energy cost of $1.883 kcal/hr Reboiler r million/year

The ammonia yield of this example (628.4 ton/day) is the greatest of the presented examples. The bottom product of this example (utilizing the SRK package) contains only CO₂ and CH₄ as opposed to Examples 1 and 2. Thus, in Example 1 where, in addition, there were also small amounts of liquid N₂ and liquid Ar, there was a relatively low bottom stream temperature (−108° C.), in comparison to the relatively high bottom stream temperature of Example 4 (−53.44° C.). 

1. A method for the separation of a mixture containing H₂, hydrocarbon, and CO₂, the method comprising introducing the mixture into a distillation column having a side stream to generate: (i) a top stream comprising H₂; (ii) a middle (volatility) stream comprising hydrocarbon; and (iii) a bottom stream comprising CO₂.
 2. The method according to claim 1 wherein the mixture further comprises N₂ which is obtained by the method in the top stream.
 3. The method according to claim 2 wherein the H₂ and the N₂ are present in a molar ratio of 3:1.
 4. The method according to claim 1 or 3 wherein the hydrocarbon comprises methane.
 5. The method according to any one of claims 1 to 4 wherein freezing of the CO₂ is substantially prevented.
 6. The method according to any one of the previous claims wherein the middle stream is obtained as a gas from a reboiler of the column.
 7. The method according to any one of claims 1 to 6 wherein the middle stream is obtained from a tray of the column.
 8. The method according to any one of the previous claims where in the mixture is introduced into a reboiler of the column.
 9. The method according to any one of claims 1 to 7 where in the mixture is introduced into a tray of the column.
 10. The method according to any one of the previous claims wherein reflux of the top stream to the column is performed using a reflux ratio between 0.001 and
 10. 11. The method according to claim 10 wherein reflux of the top stream to the columns is performed using a reflux ratio between 0.05 and
 2. 12. The method according to any one of the previous claims wherein the column has a pressure between 5 bar to a critical pressure of the mixture.
 13. The method according to claim 12 wherein the column has a pressure between 7 bar to 55 bar.
 14. The method according to any one of the previous claims further comprising cooling the mixture prior to introducing the mixture to the column.
 15. The method according to claim 14 further comprising introducing the cooled mixture into a flush to remove CO₂ prior to introducing the stream into the column.
 16. The method according to claim 15 wherein the cooled mixture is introduced into a reboiler of the column.
 17. A system for generating and separating a mixture containing H₂, hydrocarbon, and CO₂, the system comprising: (a) means for generating a mixture containing H₂, hydrocarbon, and CO₂; and (b) a distillation column having a side stream configured to receive the mixture and to generate from the mixture: (i) a top stream comprising H₂; (ii) a middle stream comprising hydrocarbon; and (iii) a bottom stream comprising CO₂.
 18. The system according to claim 17 wherein the means for generating the mixture further generates N₂ obtained in the top stream.
 19. The system according to claim 18 wherein the means for generating the mixture generates a mixture containing H₂ and N₂ in a ratio of 3:1.
 20. The system according to claim 17 or 19 wherein the hydrocarbon comprises methane.
 21. The system according to claims 17 to 20 wherein freezing of the CO₂ is substantially prevented.
 22. The system according to any one of claims 17 to 21 wherein the middle stream is obtained as a gas from a reboiler of the column.
 23. The system according to any one of claims 17 to 21 wherein the middle stream is obtained from a tray of the column.
 24. The system according to any one of claims 17 to 23 where in the mixture is introduced into a reboiler of the column.
 25. The system according to any one of claims 17 to 23 wherein the mixture is introduced into a tray of the column.
 26. The system according to any one of claims 17 to 25 wherein reflux of the top stream to the column is performed using a reflux ratio between 0.001 and
 10. 27. The system according to claim 26 wherein reflux of the top stream to the columns is performed using a reflux ratio between 0.05 and
 2. 28. The system according to any one of claims 17 to 27 wherein the column has a pressure between 5 bar to a critical pressure of the mixture.
 29. The system according to claim 26 wherein the column has a pressure between 7 bar to 55 bar.
 30. The system according to any one of claims 17 to 29 wherein the mixture is cooled prior to introducing the mixture to the column.
 31. The system according to claim 30 wherein the cooled mixture is introduced into a flush to remove CO₂ prior to introducing the mixture into the column.
 32. The system according to claim 30 wherein the cooled stream is introduced into a reboiler of the column.
 33. The system according to claim 18 or 19 further comprising means for generating ammonia from the top stream.
 34. The method according to claim 2 or 3 further comprising generating ammonia from the top stream.
 35. The method according to claim 34 wherein the hydrocarbon is methane.
 36. The system according to claim 33 wherein the hydrocarbon is methane. 